Resistive elements of interest to this invention include, for example, NTC thermistors. For example, Patent Document 1 discloses, as a composition constituting an element main body for a NTC thermistor for temperature compensation use or a thermistor for inrush current suppression (power thermistor), an oxide composition containing at least one of manganese, copper, calcium, cobalt, or nickel with borosilicate glass added thereto.
The thermistor materials which have a Mn—Co based spinel structure are widely used in conventional thermistors for temperature compensation or thermistors for inrush current suppression.
In general, circuits as shown in FIG. 4 are used for inrush current suppression. FIG. 4 shows, as a block diagram, an electrical device including a power thermistor for inrush current suppression.
Referring to FIG. 4, an electrical device 11 includes a load circuit 13 driven by an alternating-current power supply 12, and the alternating-current power supply 12 is adapted to supply power through a rectifier 14 to the load circuit 13. A power thermistor 16 for inrush current suppression is connected in series with a power supply line 15 for this power supply. In addition, a smoothing capacitor 17 is connected in parallel to the load circuit 13.
Conventionally, an NTC thermistor is often used as the power thermistor 16. The NTC thermistor exhibits, unlike common solid resistors, a high resistance from power-off immediately after power-on, and undergoes a decrease in resistance by self-heating after the power-on. Therefore, the NTC thermistor has the advantage of being able to reduce the power consumption, as compared with common solid resistors which undergo almost no change in resistance value depending on temperature changes.
To explain the operation of the circuit shown in FIG. 4 more specifically, (1) the inrush current generated by quickly charging the smoothing capacitor 17 in the case of applying power from the alternating-current power supply 12 is suppressed by the initial resistance R25 (resistance value at 25° C.) of the power thermistor 16 composed of the NTC thermistor; (2) after a steady current flows through the load circuit 13, the power thermistor 16 undergoes a decrease in resistance value by self-heating; and (3) the reduced resistance of the power thermistor 16 can reduce the power loss when the steady current flows, as compared with solid resistors, and as a result, the power consumption can be restrained.
Therefore, the increased difference between the standby (power-off) resistance value at room temperature and the resistance value obtained when the steady current flows (B constant increased) achieves a more beneficial inrush current suppression effect in regard to the power thermistor 16, and makes it possible to further restrain the power consumption in the steady state.
The power thermistor is widely used in power supply devices such as AC adapters. A rated current is constantly applied to the power thermistor in a steady state, and the temperature of the power thermistor thus reaches such a high temperature over 170° C., depending on the magnitude of the current. For example, it is due to the heat generation of the power thermistor that the whole of an AC adapter gets hot.
When the power thermistor reaches high temperatures, problems are caused such as a wiring substrate with the thermistor mounted undergoes a color change or releases an unusual odor due to the heating, or the increased temperature of the whole of the power supply device such as an AC adapter increases the load on other electronic components or causes failures. In addition, the power thermistor is inserted in series with a power-supply line, and the increased residual resistance in an on-state (a state of steady current flowing) thus not only increases the power consumption, but also causes the problem of fluctuation in steady current due to temperature fluctuation. Thus, resistive elements have been strongly desired which have lower residual resistance and low resistance temperature dependence in an on-state.
However, existing spinel thermistor materials as described in Patent Document 1 have difficulty in solving the problems mentioned above.
In general, insulators and semiconductors exhibit NTC (negative temperature coefficient) characteristics of resistance change with the increase in temperature, have a tendency to undergo a substantial change in resistance with respect to temperature as the resistivity is increased, and undergo a decrease in temperature dependence as the resistivity is decreased because of being closer to metals. More specifically, the B constant is increased as the resistivity is increased, whereas the B constant is decreased as the resistivity is decreased.
Therefore, the use of a high B-constant material can increase the difference in resistance between an off-state and an on-state, and high-resistivity materials are thus believed to be used favorably. However, the resistivity is excessively increased in that case, to make it impractically necessary to create an extremely thin element including an extremely large electrode in the case of creating a power thermistor with a resistance value slightly less than 10Ω. Therefore, existing power thermistors have no choice but to select, as thermistor materials, materials which have relatively low resistivity and the B constant on the order of 3000, and it is difficult to solve the problems mentioned above because the materials exhibit almost the same B constant in a temperature range on the order of −50° C. to 200° C.
Accordingly, a novel material is required which can, at the very least, satisfy two conditions of relatively low resistivity and high B constant, which are difficult to achieve a balance in order to solve the problems mentioned above.